A variation on heat quenching for an aged park appearance.

I bought a cheap but functioning carbine from a friend. It had been rebuilt with a lot of repro parts, one of which was a shiny black barrel band. I decided to take a shot at refinishing it.

Parked would look too new so I blasted the band with 120 grit. A very dilute solution of first Oxpho and then dilute SB gave me good control over color. Still far from a match to the barrel patina and it rubbed off pretty easily.

I decided to experiment. I left the grey Oxpho/SB base treatment on the band, put it into the toaster oven at 400F for 20 minutes (it turned white?) thinking it might harden the result. Then I got a wild hair and sprayed it while hot until it was soaked with WD-40. While steaming and sputtering the band immediately turned a dark grey/brown/green like aged park. I was amazed that the resultant finish was a perfect match in shade and flatness to the barrel. It doesn't rub off even with a lot of force.

Red hot metal quenching with heavy oil to blue is well known but since the temp was relatively low and the WD-40 was not plain oil I'm curious how it worked. Any refinishers in here that can shed some light on what happened chemically?

WD-40: Petroleum Naphtha, Petroleum Oil Distillate, and a corrosion inhibitor
Oxpho Blue: Selenious Acid, Phosphoric Acid, Copper Sulfate, Nickel Sulfate
(Super Blue also has Selenious Acid and a copper component.)